1. Field of the Invention
The present invention relates to a liquid crystal display apparatus for projecting a picture on a screen with use of a liquid crystal display element, such as a liquid crystal projector, a liquid crystal television, a projection type display device, and so on, and also relates to an optical device for use therein.
2. Description of Related Art
Conventionally, in a display apparatus for projecting a picture on a screen with use of a liquid crystal display element, upon the liquid crystal display element of such as a liquid crystal panel or the like, a light is irradiated from a light source, such as a light bulb, etc., and the light penetrating or transmitting through it is projected on the screen. By adjusting or regulating an amount or quantity of polarization at each pixel in the liquid crystal element, the display is performed or accomplished.
In such a liquid crystal display apparatus, upon a requirement from a user of obtaining a good projection picture even in a bright circumstance or place, conventionally, an attempt was made to increase the optical output of the projected picture by enhancement in a brightness of the light source and/or by improvement in the utility efficiency of the light beam thereof. For instance, with a technology described in Japanese Patent Laid-Open No. Sho 63-197913 (1988), there are utilized a polarization conversion element which comprises a polarized light separation means for dividing or separating an irregular polarized light from the light source into two (2) polarized lights that are orthogonal to each other, and a polarization direction rotating means for rotating one of the polarized lights separated into a direction consistent with that of the other polarized light, thereby improving or increasing the utility efficiency of the light.
In the liquid crystal display apparatus as mentioned in the above, within the light emitted from the light source, the light except that being projected finally is absorbed into the liquid crystal display element and optical elements and so on, in the periphery thereof, and generates heat. Therefore, the liquid crystal display element and the optical elements and so on in the periphery thereof are heated therewith. In particular, the increase in the output of the projection light, for purpose of increasing the brightness of the light source and/or of improving the utility efficiency of the light, results in the increase in light amount being incident upon the liquid crystal display element, and also leads to the increase in the heat generation of the liquid crystal display element and polarizing plates. Further, in a case where such a polarization conversion element is used within an optical system reaching from the light source until the polarizing element at an incidence side, because the polarized lights are aligned equally in the polarization direction thereof, the amount of heat generation becomes large in the polarizing element at a light exit side when black is displayed totally all over the display.
On the other hand, the liquid crystal display element is, in general, constructed with a semiconductor driving element and optical function parts or elements, such as liquid crystals, and so on. For keeping liquid crystals functioning normally, they must be kept at a temperature lower than a predetermined value (less than 60.degree. C., for example). Therefore, cooling is necessary for the liquid crystal display element. As a cooling method for this kind, there have been made many proposals.
As a conventional art relating to the cooling of the liquid crystal display element, for instance, an example which is described in Japanese Patent Laid-Open No. Hei 3-174134 (1991) is already known. In this conventional art, one of a pair of polarizing plates and the liquid crystal panel as the liquid crystal display element are disposed, being closely contacted with a cooler which contains a cooling liquid hermetically therein. The cooler is hermetically constructed with a frame and two (2) pieces of transparent plates which close or cover on both side surfaces of the frame, and the polarizing plate mentioned above and the liquid crystal panel are closely contacted with the transparent plate, respectively. A part of a heat pipe is inserted into the cooler, and another portion protruding outside the cooler is provided with radiator fins. The heat in the liquid crystal panel and the polarizing panel is transferred to the cooler, further passing through the cooling liquid and the heat pipe, and then it is radiated from the radiator fins to the outside of the cooler.
Further, in the display apparatus described in the above, a part of the light passing through the liquid crystal display element sometimes may happen to be reflected by the optical element, such as a projection lens, behind the liquid crystal element, thereby coming back to the liquid crystal display element. Such reflection light comes to be a cause of lowering in quality of the projection picture, in particular, such as a decrease in contrast thereof.
As a conventional technology for suppressing the decrease in the contrast due to the reflection light from the optical element behind the liquid crystal display element, other than a measure of improving the transparency (or transmission coefficient) by coating the optical element, there is known another technology which is described in Japanese Patent Laid-open No. Hei 6-110055, for example. With this conventional technology, a so-called .lambda./4 plate is positioned between the polarizing plate at the light exit side and the projection lens. The light penetrating through the liquid crystal panel and the polarizing plate at the light exit side reaches through the .lambda./4 plate to the projection lens. A portion of this incident light is reflected by the projection lens and comes back in the direction of the liquid crystal panel. At this moment, the reflected light has passed through the .lambda./4 plate two (2) times after passing through the polarizing plate at the light exit side. Therefore, it is rotated by 90 degrees in the polarization direction thereof, comparing to passing through the .lambda./4 plate previously. Therefore, the reflection light is unable to pass through the polarizing plate at the light exit side, thereby being absorbed totally therein.
With the conventional technology for protecting the liquid crystal display element from the heating, since one of the polarizing plates and the liquid crystal display element are positioned so that they are closely contacted with the transparent plates of the cooler which contains the cooling liquid hermetically therein, there are two (2) pieces of the transparent plates and the cooling liquid between the polarizing plate and the liquid crystal display element. And, since there occurs reflection due to a change in the refractive index on each boundary surface, the penetration light is easily attenuated therewith.
Further, with this conventional technology, since it is constructed in such a manner that the heat generation from the liquid crystal display element is radiated through a large number of heat transmission routes, i.e., the transparent plate cooling liquid heat pipe radiator fins peripheral atmosphere (outside air), it is easily affected in the cooling efficiency thereof by an integration or summation of thermal resistance, and is complex in the structure thereof. In particular, for achieving high brightness, there is a necessity of increasing the efficiency in heat radiation; therefore, a large heat pipe and/or large radiator fins are needed.
While, in the above-mentioned conventional technology for suppressing the decrease in the contrast, since it has such a construction that the reflection light from the projection lens is absorbed with the polarizing plate at the light exit side, that polarizing plate at the light exit side is heated up more easily. This heating-up is a cause of thermal deterioration (including deformation) of the polarizing plate at the light exit side. Further, additional provision of the .lambda./4 plate brings an increase in the number of the optical parts and complexity in the structure thereof.